I might be misunderstanding your question - do you want to build a new array of records which match in the nth column? I would probably do this instead (and being explicit about where the variables come from always helps me later when I'm banging my head against the wall).

for my $rec ( @recs ) {
  my @rec = split(/:/,$rec);
  next unless @rec;
  push (@goodrecs,$rec) if( $rec[$n] =~ /$pattern/ );
}
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However, if you wanted to write grep statement to achieve your wish of matching the nth field (which I assume is delimited by ':' this should work (of course where $n and @recs are defined ahead of time).

my @goodrecs = grep { my @rec = split(/:/,$_);
                      $rec[$n] =~ /$pattern/; } @recs;
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